372 FATTY SUBSTANCES (GENERAL METHODS)

and serves especially to distinguish castor oil from other oils or fats from waxes,
and may also confirm the purity or otherwise of a fatty oil.

4. Melting and Solidifying Points

The most convenient and simple method for determining these constants
is as follows : Into a thin-walled glass tube blown to a small bulb A at the
middle (see Fig. 51) sufficient of the fused fat is sucked to fill about one-
half of the bulb. When the fat has set, the branch b of the tube free from
fat is bent into a U-shape in a small flame, and the tube then set aside for
as long a time as possible (24 hours if convenient) and, if the fat melts at

a low temperature, in a cold place. It
is then attached by means of a platinum
wire or a rubber ring to a thermometer
(Fig. 52) so that the fat occupies the
upper part of the bulb, and afterwards
suspended in a beaker of water, which is
slowly heated. At a certain temperature
the fat begins to melt (at this point the
heating is discontinued) and flow down
the walls of the bulb to collect in the
lower part of the bulb. Note is made of
the temperature when the fat begins to
melt and again when it is all collected
in the bottom of the bulb. These tem-
peratures represent the limits between
which the fat melts, i.e., its melting
point.

The fat is then allowed to cool slowly
and note made of the temperature when
it begins to solidify again and when it is
all solid, the solidifying point being thus
determined.

FIG. 51

FIG. 52

The solidifying point may be determined more exactly by the method
given for the solidifying point of fatty acids (see Tallow, i, Titer Test).
A similar method is used for measuring the melting points of fats liquid at
the ordinary temperature and of those which become solid at very low
temperatures, but in such cases it is necessary to cool externally with
water and ice, with ice alone, or with a freezing mixture of snow and
salt.

These methods are also used for finding the melting and solidifying
points of the free acids obtained from any fat by saponification (see 5,
below).

The melting and solidifying points, especially those of the fatty acids of
fats, serve to characterise many of the latter and to give an indication of their
purity (see later: the various tables of characters of fats). The solidifying point
is also of importance in the determination of the so-called titer of fats (see Tallow).his may be determined by means of a Westphal balance or picnometer